Journal of Hydrology xxx (2018) xxx–xxx Contents lists available at ScienceDirect Journal of Hydrology journal homepage: www.elsevier.com/locate/jhydrol Research papers The biogeochemical imprint of human metabolism in Paris Megacity: A regionalized analysis of a water-agro-food system ⇑ Fabien Esculier a,b, , Julia Le Noë b, Sabine Barles c, Gilles Billen b, Benjamin Créno d, Josette Garnier b, Jacques Lesavre e, Léo Petit b, Jean-Pierre Tabuchi f a Laboratoire Eau, Environnement et Systèmes Urbains (LEESU); AgroParisTech, École des Ponts ParisTech (ENPC), Université Paris-Est Marne-la-Vallée (UPEMLV), Université Paris-Est Créteil Val-de-Marne (UPEC): UMR MA-102, LEESU, ENPC, 6-8 avenue Blaise Pascal, 77455 Champs sur Marne cedex 2, France b Milieux Environnementaux, Transferts et Interactions dans les hydrosystèmes et les Sols (METIS); École Pratique des Hautes Études (EPHE), Centre National de la Recherche Scientifique (CNRS), Sorbonne Université: UMR7619, METIS, UPMC, Case courrier 105, 4 place Jussieu, 75005 Paris, France c Géographie-Cités; CNRS, Université Paris I – Panthéon-Sorbonne, Université Paris VII – Paris Diderot: UMR 8504, Géographie-Cités, 13 rue du Four, 75006 Paris, France d Ecole Nationale des Ponts et Chaussées (ENPC), 6-8 avenue Blaise Pascal, 77455 Champs sur Marne cedex 2, France e Agence de l’Eau Seine Normandie (AESN), 51 rue Salvador Allende, 92027 Nanterre Cedex, France f Syndicat Interdépartemental d’Assainissement de l’Agglomération Parisienne (SIAAP), 2 rue Jules César, 75589 Paris Cedex 12, France article info abstract Article history: Megacities are facing a twofold challenge regarding resources: (i) ensure their availability for a growing Available online xxxx urban population and (ii) limit the impact of resource losses to the environment. This paper focuses on This manuscript was handled by L. Charlet, two essential resources – nitrogen and phosphorus – and challenges their sustainable management in Editor-in-Chief, with the assistance of Bruno the water-agro-food system of Paris Megacity. An in-depth analysis of the nitrogen and phosphorus Tassin, Associate Editor imprint of Paris Megacity was conducted, originally centered on human metabolism through consump- tion and excretion of these two elements. Upstream, the whole agricultural production that feeds Paris Keywords: Megacity was scrutinized and nitrogen and phosphorus flows in the agro-system were fully documented. Biogeochemical imprint Downstream, the analysis of solid waste and wastewater management in Paris Megacity showed the fate Water-agro-food system Urban metabolism of nitrogen and phosphorus imported into the city. Megacity Paris Megacity appears to rely on a very complex and international agro-food system, requiring high Nitrogen cycle levels of chemical fertilizers and strongly impacting the environment through nutrient environmental Phosphorus cycle losses. On the other hand, solid waste and wastewater management appears to be mostly disconnected from the agro-food system: even if the release of nitrogen and phosphorus into the environment has lar- gely decreased in recent years, their recycling rate remains very low. This overview of the water-agro- food system of Paris Megacity suggests that an optimal management of nitrogen and phosphorus in the three subsystems (agriculture, waste management and sanitation) should be integrated within a comprehensive approach linking agriculture and urban residues. This analysis thus constitutes a ground- work on which paradigm shift scenarios of the global water-agro-food system could be constructed. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction ply of resources and the disposal of waste. The industrial era has increased this dependence and remoteness to the point that the Born of a process of sociospatial specialization, cities are charac- urban environmental impact is greater in these supply and emis- terized by the externalization of most of their metabolism – the sion areas than in the city itself (Barles, 2015). Today the imprint flows of material and energy necessary to sustain urban life and of urban environments can be found throughout the world urban functioning – and by their dependence upon various areas (Billen et al., 2012a) and for every environmental compartment, and ecosystems located outside their boundaries, for both the sup- water being one of the most impacted. Cities’ dependence upon remote areas also questions their sustainability and their ability ⇑ Corresponding author at: Laboratoire Eau, Environnement et Systèmes Urbains to face socioecological crises that could impact their metabolism (LEESU); AgroParisTech, École des Ponts ParisTech (ENPC), Université Paris-Est as a whole: climate change and extreme climate events, change Marne-la-Vallée (UPEMLV), Université Paris-Est Créteil Val-de-Marne (UPEC): UMR in geopolitical conditions, economic crises, etc. MA-102, LEESU, ENPC, 6-8 avenue Blaise Pascal, 77455 Champs sur Marne cedex 2, More than other cities, megacities are characterized by their France. huge need for material and energy (Kennedy et al., 2015), among E-mail address: [email protected] (F. Esculier). https://doi.org/10.1016/j.jhydrol.2018.02.043 0022-1694/Ó 2018 Elsevier B.V. All rights reserved. Please cite this article in press as: Esculier, F., et al. The biogeochemical imprint of human metabolism in Paris Megacity: A regionalized analysis of a water- agro-food system. J. Hydrol. (2018), https://doi.org/10.1016/j.jhydrol.2018.02.043 2 F. Esculier et al. / Journal of Hydrology xxx (2018) xxx–xxx which food and water are of utmost importance for the life of their each of these subsystems, a detailed and regionalized analysis of inhabitants. Megacities are not just bigger than most cities: their N and P flows was conducted. We aimed at qualitatively and quan- large and diverse populations, their spatial extension, the amount titatively comprehending the stakes of the biogeochemical imprint and diversity of activities that characterize them, the complexity for sustainable development of a megacity such as Paris. For the of their functioning make the organization of megacities’ metabo- sake of this study, we therefore characterized the imprint of Paris lism particularly delicate, especially regarding food and water from Megacity by the magnitude of the flows of resources (here N and the point of view of both supply and discharge through waste and P) required to sustain its food supply and the flows of wastes dis- wastewater. These have a strong impact on biogeochemical cycles. charged into the environment as a consequence of food consump- The characterization of this impact is a key to understanding tion. We also determined the spatial distribution of these flows. megacities’ metabolism and to considering change in water and food management. This makes it necessary to (i) identify the main 2. Material and methods biogeochemical flows in terms of socioecological relevance and to analyze the biogeochemical processes involved, (ii) quantify these As recommended in the early work by Baccini and Brunner flows and (iii) locate them at the different stages of their (1991), the borders of our system are defined in this section, as circulation. well as the key issues selected. Nitrogen (N) and phosphorus (P) can be considered as the most critical biogeochemical flows regarding their socioecological impact. Steffen et al. (2015) put forward nine main control vari- 2.1. Spatial and temporal frame ables of the Earth system and suggested planetary boundaries under which these control variables should stay to prevent major 2.1.1. Spatial frame shifts in the regulation of the Earth system’s stability. Along with The urban agglomeration of Paris is ranked the 25th largest city biosphere integrity, N and P flows are considered to be in the high- in the world by the United Nations (United Nations, 2014). It is the est risk zone, ahead of the climate change control variable. The largest city of the European Union and, with a population of more concern about disruption of N and P cycles has been broadly stud- than 10 million inhabitants, it is classified as a megacity. The def- ied and documented. It is of particular significance in Europe inition of a city remains controversial and the setting of its bound- where the N cycle intensity is about five times greater than the bio- aries can vary greatly depending on the definition adopted. In this spheric cycle, leading to substantial negative damage, from aquatic paper, we choose to follow the French National Institute of Eco- and terrestrial eutrophication to poor air quality and climate nomic Statistics and Studies’ (INSEE, www.insee.fr) definition of change (Sutton et al., 2011). The environmental dispersion of P is the urban unit. The main characteristic of an urban unit is that also a matter of concern regarding fertilizer and therefore food pro- the distance between two inhabited buildings does not exceed duction (Cordell, 2010). Phosphate rock has recently been added to 200 m. In this sense, Paris Megacity is composed of 412 municipal- the list of critical raw materials by the European Commission ities totaling 10,550,350 inhabitants in the official 2012 census and (European Commission, 2014). has a density of 3,700 cap/km2 (INSEE). The term ‘‘Paris Megacity” Megacities play a major role in N and P flows and depend on will be used in this paper to refer to the Paris urban unit. them. Urban dwellers’ metabolism is embedded in a complex Paris Megacity as an urban unit should be distinguished from worldwide water-agro-food system resulting in an equally com- three other perimeters that are also commonly used to define Paris, plex biogeochemical imprint. Some studies have provided an over- illustrated in Table 1 and Fig. 1: view of urban metabolism through substance flow analysis regarding N or P (Svirejeva-Hopkins et al., 2011; Færge et al., (i) the Paris city center. This is the core municipality of Paris 2001; Forkes, 2007; Barles, 2007) or considered the impact of Megacity representing 21% of its population. It is one of urban waste and/or wastewater on the environment (Morée the densest city centers in the world with more than et al., 2013).